[issue45644] Make json.tool soak up input before opening output for writing

[Inada Naoki]

Change by Inada Naoki <songofacandy at gmail.com>:


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nosy: +methane
nosy_count: 4.0 -> 5.0
pull_requests: +28860
pull_request: https://github.com/python/cpython/pull/30659

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45644>
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